Large two-stroke internal combustion engines are typically used as prime movers in large ocean going ships, such as container ships or in power plants.
These engines are typically provided with two or three fuel valves arranged in each cylinder cover. A conventional fuel valve, as shown in FIG. 1, has a longitudinal axis that is arranged roughly at an angle of 10 to 15 deg to the direction of the movement of the piston in the cylinder of the engine. The fuel valve is provided with a nozzle at its front end that projects into the combustion chamber. The nozzle is provided with axial bore and a plurality of nozzle holes that direct the fuel away from the cylinder walls and into the combustion chamber. Typically, there is a swirl in the scavenging air in the combustion chamber at the time of injection and most of the nozzle holes are directed to inject the fuel with the flow of the swirl although one of the nozzle holes may be directed to inject the fuel into the swirl.
The fuel valve is provided with a spring biased valve needle that acts as a displaceable valve member. When the pressure of the fuel exceeds a preset pressure, e.g. 350 bar the valve needle is lifted from its seat and the fuel is allowed to flow to the combustion chamber via the nozzle at the front of the fuel valve.
The maximum combustion pressure of a large two-stroke self-igniting turbocharged internal combustion engine is very high, e.g. 200 bar and it is therefore difficult under an injection event to provide fuel at a pressure that is significantly higher than the combustion pressure.
Known fuel valves for large 2-stroke self-igniting turbocharged internal combustion engines have a construction that causes the closing pressure, i.e. the pressure at which the valve needle returns to its seat to be lower than the opening pressure, i.e. the pressure at which the valve needle gets lift from its seat. This is due to the fact that the effective pressure surface that acts in the opening direction of the valve needle against the bias of a spring or other resilient means increases at the moment that the valve gets lift from the valve seat. Thus, the valve needle will not return to its seat before the pressure in the fuel valve falls significantly below the pressure at which the fuel valve opened. The resulting low-pressure at the end of the injection event can result in the fuel not being injected with sufficient pressure through the nozzle holes, thereby resulting in less than optimal combustion for the fuel that is injected during the last part of the injection event.